UPDATE The below code unlocks a bonus dueling stage, not Bonus Quidditch. If anybody knows how to unlock it, please let me know! /UPDATE
When I wrote Lunabean's Harry Potter: HBP Walkthrough there was one thing I just couldn't unlock: Bonus Quidditch.
I figured that it would unlock once I earned all crests. Nope. I then figured if I got all of the Quidditch badges it would unlock. Nope. Maybe all badges in everything? Nope. So, I searched the interwebs and what did I find? That many people were having this issue and there was no solution. WTF?
Now over a month later, an answer has surfaced. It's a code, dammit!!! I no longer have the game so I have yet to try it out, but according to various sources, it seems that if you follow the below instructions, you'll unlock Bonus Quidditch. Yay!!! (The disappointing part? Bonus Quidditch is lame.)
Go to "Rewards" (in there you will see "Badges" and "Credits" along with how many Crests you have found) and with your D-pad enter: RRDDLRLRLRS
R = Right
D = Down
L = Left
S = Start